This invention relates to improved iron base alloys having ferritic body-centered cubic micro-structures. The invention is particularly directed to tantalum modified ferritic iron base alloys having improved high temperature mechanical properties and oxidation resistance. These alloys are particularly useful in high temperature applications including furnace linings, flue stacks, and the like.
Ferritic iron base alloys, both with and without the addition of aluminum, have been available as Series 400 stainless steels, chromium-molybdenum-vanadium steels with less than 0.1 weight percent carbon and iron-nickel magnetic compositions. Aluminum added to these materials in the amount of a few percent generally assures that the alloy remains ferritic and free of damaging allotropic phase transformations. This aluminum addition further contributes to corrosion resistance as well as resistance to scaling, and the presence of aluminum is sometimes used to control grain size.
Chromium added in addition to aluminum provides resistance to oxidation and corrosion at very high temperatures up to 1,290.degree. C. Such alloys are used almost exclusively as resistor heating elements and have compositions containing chromium in excess of 23% together with 5% aluminum. Recently developed ferritic alloys with 18% chromium and 2% aluminum, as well as AISI 405 stainlss steel have received attention for high temperature applications where strength is not a requirement.
The principal disadvantage of these prior art ferritic iron base alloys is that they lost their ultimate strength and rupture strength at temperatures in excess of 650.degree. C. When present day ferritic iron-chromium-aluminum alloys are utilized at higher temperatures, the alloys lack sufficient strength to support their own weight, and they have unsatisfactory corrosion resistance when combined with even moderate loads. Thus, such alloys have not been used in high temperature applications above 800.degree. C.